As the recent demands for office automation equipment extend not only to higher speed and image quality, but also to improved durability for key components of the equipment, it is now indispensable for intermediate transfer belts to have materials designed to stably produce high-quality transfer images over an extended period of time. The development of a technique for producing an intermediate transfer belt having both excellent electrical and physical properties is becoming increasingly important. Excellent electrical properties include, for example, the ability to achieve accurate image transfers in a color image-forming unit, and the ability to prevent variations in resistance due to the transfer voltage, enabling high-quality transfer images to be stably produced over an extended period of time. Excellent physical properties include, for example, little deterioration in flatness due to plastic deformation caused by loads applied to the width direction of the belt, excellent bending durability, and stable operation even after extended use.
A carbon black-dispersed polyamic acid solution composition, used as a feedstock for such an intermediate transfer belt, is generally produced by uniformly dispersing and mixing carbon black that is added to a polyamic acid solution prepared by the polymerization of tetracarboxylic dianhydride and diamine. Polyamic acid solutions used in intermediate transfer belts generally have a high molecular weight in view of the mechanical properties and the like of the belts. However, such high molecular weight polyamic acid has only limited solubility in organic polar solvents, and its concentration cannot be increased.
Further, because the addition of carbon black to a polyamic acid solution increases the viscosity of the solution, it is difficult to grind the carbon black even with the force of impact between the balls of a disperser such as a bead mill. Therefore, the formation of a uniform dispersion of carbon black in a polyamic acid solution must involve grinding the carbon black by a disperser, and a surface phenomenon known as “wetting”, in which the carbon black being pulverized is wetted by the solvent. Currently, a uniform carbon black dispersion is obtained by adding a large amount of organic polar solvent together with the carbon black. Consequently, the solids content of the resulting carbon black-containing polyamic acid solution composition can only be increased to generally about 15 to about 20 weight %.
A polyamic acid solution composition with such a low solids content cannot be easily molded into a thick belt in a single operation. Further, since the composition contains a large amount of organic polar solvent, the evaporation and removal of the solvent takes a great deal of time. These factors add time and costs to complete the entire process, leaving room for improvement in the efficiency and economy of the process.
Various carbon black-dispersed polyamic acid solution compositions have been considered as feedstocks for intermediate transfer belts. Known, for example, are semiconductive polyimide resin belts that are produced from a feedstock solution in which a conductive filler is dispersed in a high molecular weight polyamic acid solution obtained by reacting 3,3′,4,4′-biphenyltetracarboxylic dianhydride and p-phenylenediamine (see, for example, Patent Documents 1 to 3). However, the polyamic acid solutions disclosed in these patent documents have a solids content of at most about 20 weight %, and thus, have only a limited solids content. Furthermore, semiconductive belts containing 15 weight % or more of carbon black in these polyimide resins are very brittle, having lost the inherent toughness of polyimide resins. Therefore, when such semiconductive belts are used as intermediate transfer belts, they become cracked or broken by loads applied to the width direction of the belts.
Semiconductive belts containing a copolymer of 3,3′,4,4′-biphenyltetracarboxylic dianhydride, p-phenylenediamine, and 4,4′-diaminodiphenyl ether are known as belts that retain the inherent toughness of polyimide resins (see, for example, Patent Documents 4 and 5). However, even the polyamic acid solutions disclosed in these patent documents have a solids content of at most about 20 weight %.
Rotational molding has been considered as a process for producing an intermediate transfer member made of a polyimide resin. Rotational molding is a process wherein a polyimide resin precursor solution is applied to the inner surface of a cylindrical mold, and a film is formed by centrifugal molding. Next, a portion of the solvent is removed and the precursor is subjected to partial imidization until the film becomes self-supporting. The film is then stripped from the mold, the outer surface of a tubular mold is covered with the stripped film, and the solvent is removed and the imidization reaction is completed.
As such a rotational molding process, a process has been disclosed wherein molding is performed under specific conditions such that the influence of centrifugal force in the centrifugal molding step can be reduced (see, for example, Patent Document 6). According to Patent Document 6, the reduced influence of centrifugal force in the centrifugal molding step prevents particles of carbon black and the like from concentrating in the thickness direction of the belt, thus enabling the production of a belt with a flatness of 2 mm or less. This process, however, suffers from the following problem: In the imidization step, the solvent remaining in the stripped belt gathers between the belt and the tubular mold, thereby causing the belt to expand. The shrinkage of the belt that subsequently occurs during the imidization reaction degrades the flatness of the belt, thereby causing the belt to ripple.    Patent Document 1: Japanese Unexamined Patent Publication No. 7-156287    Patent Document 2: Japanese Unexamined Patent Publication No. 10-63115    Patent Document 3: Japanese Unexamined Patent Publication No. 10-83122    Patent Document 4: Japanese Unexamined Patent Publication No. 2002-341673    Patent Document 5: Japanese Unexamined Patent Publication No. 2003-266454    Patent Document 6: Japanese Unexamined Patent Publication No. 2001-260151